Cystic-Like HER2-Positive Breast Cancer with Low Tumor-Infiltrating Lymphocytes and High Programmed Death-Ligand 1 Expression in a Young Woman: A Case Report.

Background: HER2-positive breast cancer is generally correlated with poor prognosis, whereas it demonstrates a favorable response to HER2-targeted therapies. Typically, HER2-positive tumors present as solid masses, while cystic features are exceedingly rare. Case Presentation: We present a case of a 37-year-old female with imaging findings of a large cystic mass (9.9 × 1.8 cm) in the left breast. Pathological examination confirmed grade III invasive carcinoma of no special type with HER2 positivity and HR positivity, low tumor-infiltrating lymphocytes (TILs, 5%), and high PD-L1 expression (CPS = 25%). The patient underwent a modified radical mastectomy with axillary lymph node dissection, revealing metastasis in 7 of 15 lymph nodes and the presence of lymphovascular invasion. Adjuvant therapy with the TCbHP regimen (docetaxel, carboplatin, pertuzumab, and trastuzumab) was initiated, with a total of six cycles planned, followed by maintenance therapy with trastuzumab and pertuzumab (HP) for 1 year. To date, the patient has tolerated the treatment well without evidence of distant recurrence or metastasis. Conclusion: This case underscores the discordance between radiological and pathological findings in breast cancer, highlighting the clinical significance of low TILs and high PD-L1 expression in HER2-positive tumors, and emphasizes the importance of individualized surgical and adjuvant treatment strategies.